The effect of addition of rhodium upon the characteristics and catalytic activity in carbon dioxide reforming of methane using rhodium promoted nickel catalysts supported on two commercial alumina, was studied. The number of reduced metallic atoms exposed on the surface increased for the rhodium promoted catalysts, which produced an increase in the catalytic activity for the reforming reaction. However, the specific activity measured by the turnover frequency resulted practically constant for promoted and not promoted catalysts suggesting that the increase in the number of metal surface atoms caused the activity enhancement.

Conversion of methane and carbon dioxide, which are two of the most abundant carbon-containing materials, into useful products is an important area of the actual catalytic research. The methane CO2 reforming (dry reforming) to produce synthesis gas,

CH4 + CO2 « 2 CO + 2 H2 D H = 260,5 kJ/mol

(1)

is a very attractive route to produce energy and valuable compounds. This reaction offers advantages over methane steam reforming to produce a H2/CO ratio adequate for processes such as the production of higher hydrocarbons and oxygenated derivatives. The process has also important environmental implication since both methane and carbon dioxide are greenhouse gases [1]. The use of the reaction for energy storage [2] and the possibility of transforming the CO2 without producing carbonization [3] have renewed the interest in the development of the process.

Generally is considered that dry reforming is accompanied by the reverse of water-gas shift as secondary reaction

CO2 + H2 « CO + H2O D H = 41,0 kJ/mol

(2)

The catalysts based on noble metals are reported to be adequate for the operation free of carbon accumulation compared to Ni-based catalysts [4]. However, the fact that these noble metals are expensive and of limited availability makes the development of new supports [5], and/or the addition of promoters or additives to improve the performance of catalysts based on Ni [6], a challenge to the catalytic scientific community.

It has been reported by several authors that carbon deposition could be reduced by pretreatments such as sulfur passivation of Ni catalyst [7,8]. But sulfur passivation decreases the activity of catalysts. Among other reported ways of improving the performance of such catalysts, the addition of metal additives into nickel-based catalysts could be a relatively simple and effective method to increase their resistance to coking. Borowiecki and Golebiowski [9] have reported that the addition of molibdenum and tungsten into the nickel based catalyst reduces carbon deposition while entailing no change in catalyst activity. Also Choi et al [10] have reported the additive effect of manganese on the performance of a commercial nickel-based catalyst (ICI-46-1) in CO2 reforming of methane with a particular attention to resistance to coking and high catalytic activity. Castro Luna et al [11] have also shown the positive effect on the performance of alumina supported nickel catalysts promoted with small amounts of rhodium in the steam reforming of methane.

In this work the behavior of Ni on alumina catalysts supported on commercial alumina and promoted with low rhodium concentrations in the dry reforming of methane was studied.

2. EXPERIMENTAL

2.1 Preparation and characterization of catalysts

The catalysts were prepared by aqueous incipient wetness multiple impregnation of two commercial alumina, Alcoa F110 and Kaiser A202, with a solution of Ni(NO3)2.6 H2O (Merck). After the impregnations, they were dried at 120°C for ca. 5 h and calcined at 550°C for 3 h in air. Once the nickel levels desired were obtained, they were impregnated with Rh(NO3)2.2 H2O (Johnson & Matthey), repeating the drying and calcination processes. Blank samples of supports were impregnated only with deionized water to study possible modifications caused by the aqueous sequential impregnation. The characterization of these samples did not show changes induced by impregnation with water.

The thermal behavior of catalyst supports from room temperature up to 1400°C was studied by Differential Thermal Analysis (DTA/TG). The runs were carried out at a heating rate of 10°C/min in dry air flow (50 ml/min) using a Netzsch STA 409 thermobalance.

The crystalline phases were determined by X-ray diffraction (XRD) with a Rigaku 2002 Miniflex diffractometer, equipped with a graphite monocromator with Cu-Ka radiation (l =1.5418 Å). When possible, the nickel crystallite size was calculated using the Ni(111) reflection and the Scherrer formula corrected for instrumental line broadening.

Total surface area was measured by N2 adsorption at 77 K using a Micromeritics Digisorb 2600 instrument. Specific surface area (SBET) was calculated according to the BET equation and an effective cross-sectional area of 16.2 Å2 for N2. The pore volume (Vp) was calculated from the amount of adsorbed nitrogen at relative pressures close to saturation.

Transmission Electronic Micrographs (TEM) were conducted with a JOEL 100 CX system, with a high tension voltage of 100 kV, and a magnification of a 50000X, to investigate the surface morphology and crystallinity of the reduced and used catalysts. The resolution of this instrument is of 6 Å. Moreover particle size distribution was obtained to calculate the number-weighted (dn = å2)ni di / åni), volume-weighted (dv = åni di4 / ånidi3) and a surface-weighted (ds = åni di3 / ånidi2 ) particle sizes, in order to compare with the ones calculated from XRD.

2.2 Activity tests

Methane CO2 reforming was carried out in a fixed bed flow reactor. In the feed section, the reactants were led through a set of Brooks 5876A mass flow controllers. A quartz reactor (12 mm I.D.) was used heated by a furnace with three independent heating sections, each with its own PID temperature controller, facilitating isothermal operation. The temperature profile along the bed was measured by a sliding K-type thermocouple placed in a thermowell situated at the central axis of the reactor.

The reaction zone containing the catalyst and inert solid diluent has a length of 5 cm. Pure a -alumina spheres were used in the preheating section and the section after the catalyst bed. Methane (99.99%), and hydrogen (99.9%) were supplied by LAir Liquide and carbon dioxide (99.9%) by Argon.

Typically 0.2 g of catalyst powder diluted with inert alpha alumina were reduced in situ in flowing H2 (30 cc/min) at 550°C during 12 h prior to reaction. Once the selected operation conditions were obtained, samples were taken at intervals of 30 min and analyzed by on-line gas chromatography (Shimadzu GC3BT) using a thermal conductivity detector and a gas sampling valve. A 2 m Carbosphere column and Ar as carrier gas was used to achieve separation of hydrogen, methane, carbon monoxide and carbon dioxide. Data were processed with a Spectra-Physics 4600 Integrator.

DTA/TG curves of figures 1 and 2 show a similar thermal behavior; an endothermic transformation around 100°C and an exothermic one above 1100°C. The former corresponds

Fig. 1. DTA/TG diagram of the Kaiser support.

Fig. 2. DTA/TG diagram of the Alcoa support.

to the loss of free water and the latter to the transformation of transition alumina to a stable alpha alumina phase. In the case of the Alcoa support, an endothermic peak also appears around 420°C corresponding clearly to the boehmite transformation into gamma alumina. On the other hand, these alumina phases were also detected by X ray diffraction (Table 1). The diagram for the Kaiser support did not show this endothermic transition in spite of the boehmite presence detected by X ray diffraction (Table 1), however, a weight loss was recorded at the temperature corresponding to the transformation.

Table 1. Evolution of physical properties during preparation and use of catalysts

SUPPORT

SAMPLE

DRX

SBET (m2/g/)

Vp (cm3/g)

SUPPORT

B + G

342

0.427

BLANK 1

G

214

0.431

BLANK 2

G

201

0,431

KAISER

UNRED. CAT.

G + NiO

229

0.356

RED. CAT.

G + Ni0

182

0,385

USED CAT.

G + Ni0

155

0.367

SUPPORT

B + A

208

0.284

BLANK 1

G + A

148

0.307

BLANK 2

G + A

127

0.305

ALCOA

UNRED. CAT.

G + A + NiO

170

0.279

RED. CAT.

G + A + Ni0

140

0,283

G: g -alúmina, B:boehmita, A: a -alúmina

3.2 Microstructure and phase characterization

Supports and unreduced, reduced and used catalysts were characterized according to the evolution of their microstructure during the preparation, pretreatment and use of such materials in the dry reforming of methane. Results for the catalysts with only nickel content are shown in Table 1

The Kaiser support showed to be composed by a mixture of boehmite and g -alumina and the Alcoa support was composed by a mixture of boehmite and a -alumina. In both cases, the boehmite was transformed into g -alumina during the preparation of the unreduced catalyst and they did not experiment other transformation of the crystalline phase during reduction and use of the catalyst. A complete reduction of the original NiO to metallic Ni was observed in the XRD of the reduced catalyst. The absence of rhodium diffraction peaks in the XRD spectra of the Ni-Rh catalysts is probably due to poorer XRD resolution at relatively low rhodium loadings.

In both supports the transformation of boehmite to g -alumina during the preparation of the non-reduced catalyst is accompanied by a significant decrease of the surface area (more noticeable in the Kaiser support), figures 3 and 4, coming from the microporosity disappearance present in the boehmite and, apparently, the nickel presence seems to retard this effect.

Fig. 3. Surface area evolution

Fig. 4. Pore volume evolution

3.3 Catalysts particle size

From the micrographs TEM of the reduced catalysts, it was possible to determine a particle size distribution, from which the mean particle size was calculated. In Fig. 5 the results for the catalysts with only nickel content and the ones with the highest rhodium content, for both series are shown. As can be seen the rhodium addition does not produces a great effect on the particle size distribution.

Fig. 5. Particle size distribution: A: CA1, B: CA4, C: CK1, D: CK4

In Table 2 are also shown surface- and volume-weighted particle size. Particle sizes estimated through XRD line broadening measurements are also provided for comparison, showing reasonably agreement with the values obtained from TEM.

Table 2. Particle size for Ni and Ni-Rh catalysts.

Catalyst

Particle size (nm)

XRD

TEM

dv

dv

ds

dn

CK1

16

17.9

17.1

15.3

CK4

16

18.6

17.8

16.2

CA1

16

19.2

18.6

17.4

CA4

18

17.9

17.4

16.2

The micrographs TEM of the used catalysts (not shown) do not supply evidence of whisker carbon formation during the reforming of methane reaction. The comparison of the particle size distribution of the used catalysts shows not only an absence of coke but also lack of substantial sintering of nickel particles during the reaction.

3.4 Catalytic activity

For all the species tested, there was a transient period of at least 4-6 h where activity changes about 30 %, followed by a constant level of activity, which was quoted in Table 3, no deactivation occurring for more than 50 h on line. Table 3 shows the results for the methane and carbon dioxide conversion, XCH4 and XCO2, the H2/CO molar ratio in the product, the consumed reactants ratio, CH4/CO2, the CO selectivity, the percent metallic dispersion of the used catalyst, D, and the turnover frequencies, TOFCH4 and TOFCO2, of the catalysts studied. Equilibrium conversions for reference conditions are: XCH4 (eq.) = 0.17 and XCO2 (eq.) = 0.38.

In all cases XCO2 was higher than XCH4 due probably to the effect of the water-gas shift secondary reaction. For both catalyst series, XCH4 and XCO2, increase with the rhodium content reaching practically equilibrium conversion for the catalysts with the higher rhodium content. Catalysts with just rhodium on both supports showed very low conversions (< 1%) in the same conditions, and were not included in the table. These suggest that a synergistic effect exists between Ni and Rh in the range of atomic ratio studied. An increase in the H2/CO ratio was also found, probably due to the steam reforming reaction occurring between methane fed and water from reverse of water-gas shift.

Dispersion data, i.e., Nisurf /Nitotal , calculated from carbon monoxide uptake indicated a low degree of nickel dispersion for all of the catalysts. Dispersions show also a redispersion effect caused by rhodium addition. Blank tests make sure that the effect is not due to thermal treatment.

The relation between activity and dispersion, indicates that the activity increases with dispersion. The catalytic activity is determined apparently by the added amount of Rh as demonstrated in Table 2. The Rh-Ni catalysts have much higher activity than would be expected from the linear combination of the base catalysts and the corresponding Rh catalysts.

TOF values reported in Table 3 for unpromoted and promoted catalysts can be considered essentially the same. Both observations suggest that Rh promotion increases the reaction rate by increasing dispersion, thereby increasing the number of reaction sites. This additional sites appear to be identical to those present in the absence of Rh.

Table 3. Catalytic activity

Catalyst

XCH4 (%)

XCO2 (%)

H2/CO (%)

CH4/CO2 (%)

SCO (%)

RCO (%)

D (%)

TOFCH4 (s-1)

TOFCO2 (s-1)

CK1

11.0

30.9

24.2

35.6

84.6

17.7

4.0

0.4

1.1

CK2

12.8

32.0

32.1

39.9

86.4

19.3

5.2

0.4

0.9

CK3

14.2

35.3

37.2

40.2

83.6

20.7

6.3

0.3

0.8

CK4

17.1

37.0

44.1

46.2

87.7

23.8

7.4

0.4

0.9

CA1

8.3

26.4

16.5

31.3

81.9

14.2

4.0

0.7

2.1

CA2

9.9

31.0

31.2

31.9

73.7

15.0

7.0

0.4

1.4

CA3

11.9

32.7

34.7

36.5

79.5

17.7

10.4

0.4

1.0

CA4

14.0

34.2

41.0

41.0

82.3

19.9

11.4

0.4

1.0

PT = 1.0 atm, T = 550 ° C, W/FCH4 = 0.5 g h/mol

The correlation between the activity and the metallic dispersion measured by the adsorbed CO amount indicates that the activity increases with the dispersion. However, the rhodium addition, practically, does not have influence on the specific rate expressed as turnover frequency, suggesting that the activity increase is predominantly caused by the increase of the number of sites rather than by the electronic effect.

4. CONCLUSIONS

The observations allow to conclude that the addition of small amounts of rhodium to the nickel catalysts supported on alumina produces an increase in the amount of metallic atoms exposed on the surface determined by CO chemisorption. The increase in the metallic surface area is accompanied by an increase of the methane reforming rate, maintaining almost constant the turnover frequencies. From the actual results, it is possible to infer that the rhodium acts only as a promoter of the dispersion of nickel producing an increase in the number of active sites for the reforming. Similar results have been observed previously for Ni/Al2O3 catalysts promoted with Rh in methane steam reforming [11], and for Co/Al2O3 catalysts promoted with Ru in the Fischer-Tropsch reaction [12].